Thanks for the additional information. I quickly looked at a ETM today but it was for a newer model than yours. (1986 ETM)The only think that I see that these circuits share is the ground point W1. You problem sounds very unusual and could be caused by a bad ground. W1 is located behind the Inst. Cluster. If this ground is good The next step to to check each of this circiuts completly. You will need a OEM ETM and a good multimeter for this.
Good Luck